There are many amusement games known which are characterized by the presence of a surface, or playfield, on which a ferromagnetic projectile such as a steel ball moves. The projectile may be given an initial impulse by a player from a lower to higher point of a playfield, where it then ricochets from a stop and moves back down the playfield in a random-appearing manner, bouncing off posts and other obstacles. In another variety of such game, a steel ball may be aimed and projected at a target point of a planar playfield that is substantially level with respect to the local gravitational vector. In yet another example of such game, the playfield may have substantial deviations from flatness; for example, it might be bowl-shaped. The foregoing are given by way of example only and are not intended to be exhaustive of the class of games considered herein. Other games may be considered in which the projectile has a free fall trajectory during at least part of its travel.
In any of the games considered it may be desirable to detect the position of the moving ferromagnetic projectile without perceptibly affecting the motion of the projectile across the playfield. A pinball game, for example, may have a special alley for a ball to pass through unimpeded, but if the passage occurs at certain times a bonus score is awarded. The active, or bonus periods, might be indicated by lighting of appropriate indicator lights. Alternatively, in games played with aimed projectiles on a substantially flat level surface, it may be desirable to have sequences of moving target points at which steel balls may be fired and scores awarded if the ball rolls over the moving target points. Such games, which are known as "gun games", may be made to operate very much like video games that feature targets which may be shot at and "destroyed" by a player.